The innate immune system detects the presence of microbes in tissue by pattern recognition of conserved microbial structures, known as pathogen-associated molecular patterns (PAMPs). However PAMPs can be present in all microbes regardless of their pathogenic potential. To distinguish pathogens from other microbes with lower disease-causing potential, the innate immune system can detect pathogen-induced processes, such as the presence of microbial products in the host cell cytosol, through mechanisms that are not fully resolved. Identification of signaling pathways involved in the detection of pathogen-induced processes is often difficult, because PAMPs expressed by a pathogen can activate many pattern recognition receptors in parallel. Brucella abortus is a stealthy pathogen expressing modified PAMPs that no longer serve as agonists for pattern recognition receptors. As a result, host responses generated during B. abortus infection are entirely dependent on detecting the deployment of a virulence factor, the type IV secretion system (T4SS), as a pathogen-induced process. Here we propose to use this model organism to define a new signaling pathway involved in sensing the T4SS-dependent injection of proteins into the host cell cytosol as a pathogen-induced process. The objective of this application is to study how translocation of the T4SS substrate VceC into host cells induces pro-inflammatory responses and alters the disease outcome. Our central hypothesis is that translocation of the T4SS substrate VceC activates the unfolded protein response (UPR) with consequent induction of NF-kB- dependent inflammatory responses, thereby contributing to B. abortus-induced inflammation in vivo. We will test our hypothesis by identifying the innate immune signaling molecules downstream of the UPR that activate inflammatory responses and by testing the role of these pathways in vivo. Successful completion of this work will move the field forward by establishing the UPR as a component of the innate immune system that detects microbial proteins targeting the ER as a pathogen-induced process. This new concept has important ramifications not only for bacterial pathogenesis, but also for viral pathogenesis, innate immunity and the pathogenesis of certain inflammatory disorders involving the UPR, such as type 1 diabetes and inflammatory bowel disease.